Brilliant aspen trees facing decline

The explosion of color in the high country this fall is overshadowing a hangover some aspen trees are suffering from the severe drought earlier this decade.

A variety of factors have combined in recent years to kill concentrated patches of Colorado’s vast aspen forests, including some in the Aspen area, U.S. Forest Service researchers discovered. They labeled the problem as sudden aspen decline, or SAD.

It appears that dry conditions and high temperatures at the start of the decade placed some aspen trees under extraordinary stress, said Jim Worrall, a forest pathologist with the U.S. Forest Service’s Forest Health Management office in Gunnison. That made them more susceptible to fungi and insects.

The one-two punch has killed older trees in open stands on south- and southwest-facing aspects, Worrall said. Aspens thrive between 6,800 and 11,150 feet in elevation, according to the Forest Service. Most of the trees SAD has killed are at the lower end of that range, he said.

Worrall said he hasn’t visited the aspen forests in Aspen to check the health of forests there, but he checked aerial surveys the agency is compiling around the state. Those images indicate that SAD has killed patches of aspen in the Castle Creek and Maroon Creek valleys as well as about five miles east of Aspen on the north side of Highway 82.

McClure Pass, south of Carbondale, also has been affected, but the vast aspen groves along Kebler Pass have not, Worrall said.

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There is no estimate of how many acres of aspen that SAD has killed in the upper Roaring Fork basin. Statewide about 140,000 acres of aspen had died as of 2006, the Forest Service said. The estimate will be updated with the latest aerial surveys. Areas around Paonia and in the San Juan Mountains in the southwest part of Colorado are among the hardest hit.

A Forest Service website on SAD explains how it is different from normal dieoffs that forests experience.

“The change is evident on a landscape scale, as opposed to the individual stand-level changes we have typically seen in the past,” the website said. “The current phenomenon has increased dramatically over a few years, as opposed to the typical changes that we see over decades.”

The uniform age of many aspen stands contributes to that widespread death. SAD also is different from regular forest evolution because it kills root systems, Worrall said. Typically, when old aspens die, they regenerate from the root system. That’s not happening with this sudden decline, he said.

It’s unknown how many acres of aspens will eventually die from SAD. Cooler, wetter weather this summer may have given aspens a break. However, it was probably too late to help the trees that were already stressed and attacked by insects and pathogens.

The good news is that there are millions of acres of healthy aspens, and many of them are putting on their annual show of colors right now.